Due to low formability of conventional high strength steel strips developed for application to structural members and interior sheets of automobiles, it is difficult to apply such high strength steel strips to components of complicated shapes.
Conventionally, automobile manufacturers have focused on simplification of the complicated shapes or separate machining of several discrete pieces constituting a single complicated component.
However, formation of the complicated components with separate machining of the several discrete pieces requires a secondary welding process and provides significant limits in design of an automobile vehicle body due to differences in strength between a welded part and a matrix.
Thus, the automobile manufacturers have continuously demanded high strength and high formability materials that can be applied to such complicated components and increase a freedom of design with respect to the automobile body.
In particular, it has been increasingly demanded with the aim of enhancing fuel efficiency and reducing air pollution to provide high strength steel strips that have superior formability and can reduce the weight of automobiles.
As to conventional steel strips for automobiles, high strength low carbon steels with ferritic matrix structure have been used in consideration of the formability.
However, in the case where the high strength low carbon steels are applied to automobile steel sheets, steel having tensile strength of 800 MPa or more fails to ensure elongation up to 30% or more in commercial applications.
As such, since it is difficult to apply the high strength steel at the level of 800 MPa or more to the automobile components of the complicated shapes, such high strength steel also requires simplification of the shapes and fails to ensure the freedom of design for the components. To solve such problems, novel steels have been studied, and some examples thereof can include high-ductility, high-strength and high Mn austenitic steels as disclosed in JP1992-259325 and WO 02/101109.
For the high Mn steel of JP1992-259325, although ductility can be secured by addition of Mn in a large amount, a deformed part of the high Mn steel experiences severe work hardening. As a result, it has a shortcoming in that steel strips thereof are likely to experience fracture after machining.
For the high Mn steel of WO 02/101109, although the ductility can also be secured, it has shortcomings in terms of electroplating and galvanizing properties due to addition of silicon in a large amount.
Moreover, since other types of steel contain large amounts of Mn, they have a shortcoming in that annealed products thereof have significantly low corrosion resistance.
Therefore, it is an object of the present invention to provide a steel strip that has superior formability, high strength, superior surface properties and plating characteristics, and a plated steel strip using the same.
It is another object of the present invention to provide a steel strip that not only has superior formability, high strength, superior surface properties and plating characteristics, but also is relieved in sensitivity to generation of cracks, and a plated steel strip using the same.